1. Field of the Invention
The present invention relates to a controller and a control system for an electric motor, which drives a mechanical element of an industrial machine, such as a machine tool, and in particular, relates to a controller and a control system, which estimates the inertia of an article to be driven by the electric motor.
2. Description of the Related Art
In a machine tool or the like, it is important to know the inertia of an abject to be driven by an electric motor for determining a process condition or the like, and for controlling the motor with high accuracy. For example, when a time constant of an acceleration/deceleration command as the process condition is to be determined, it is advantageous to correctly know the inertia in order to stably control the motor and improve the accelerating/decelerating performance of the motor. Further, it is also advantageous to correctly know the inertia for calculating the responsiveness of the speed control in a control system.
However, it is not easy to determine the inertia of the article to be driven. For example, it is difficult to calculate the inertia of an article to be driven if the article has a complicated structure. In addition, if the inertia of the article driven by the electric motor varies due to attachment/detachment of a workpiece or the like, it is advantageous to know the inertia in each case. However, it is not easy to determine the inertia in a short period of time.
On the other hand, it is a known technique to calculate the inertia by using current feedback “I” and acceleration feedback “a” during operation of the motor. For example, inertia J can be calculated by the following equation where Kt is the torque constant of the motor.J=I·Kt/a 
Japanese Unexamined Patent Publication No. 8-140386 discloses a method for estimating the inertia, in which an electric motor is activated by an acceleration/deceleration command and the inertia is estimated from a feedback signal during operation of the motor. Further, Japanese Unexamined Patent Publication No. 6-82346 discloses a method for estimating the inertia by a feedback signal during operation based on an arbitrary command.
The above prior art may include some improvements as described below.
The first improvement is the operation range and operation time required to estimate the inertia. As described in Japanese Unexamined Patent Publication No. 8-140386, when the inertia is estimated based on a feedback signal during operation of an electric motor by an acceleration/deceleration command, it is necessary to estimate the inertia after a generated torque is stable in relation to the acceleration/deceleration command. Therefore, it is necessary to operate the electric motor for a relatively long period of time. In particular, in a machine tool, the operation range of a drive axis may be limited, whereby it may be difficult to obtain the operation range sufficient to estimate the inertia. In addition, when a workpiece is frequently replaced with another, it may be difficult to obtain sufficient time for carrying out the particular operation required to estimate the inertia.
The second improvement is stability of controlling the electric motor during estimation of the inertia. In the inertia is measured after the inertia is changed by attaching a workpiece, it maybe difficult to stably control the motor by a control gain before the inertia is changed.
The third improvement is variability of estimating the accuracy of the inertia. As described in Japanese Unexamined Patent Publication No. 6-82346, when the inertia is estimated when an electric motor is operated by an arbitrary command, the estimating accuracy may be varied depending on the kind of operation.
The fourth improvement is influence of noise. When the inertia is estimated, current feedback or speed feedback is used, which includes some noise. Therefore, such noise may deteriorate the accuracy of the inertia.